Anti-skid brake control system

Abstract

An anti-skid brake conrol system includes a sensor which produces a signal proportional to the acceleration of a braked wheel and a comparator that responds to the acceleration signal by producing a signal which undergoes a step change at set acceleration levels so as to divide successive deceleration/acceleration cycles into successive acceleration periods separated by intermediate periods. The output of the comparator is integrated to produce an output signal which changes in a progressive manner in one sense during each acceleration period and in the opposite sense during each intermediate period so as to vary in accordance with surface friction conditions. The output signal is used to vary a control parameter or function within the system in accordance with the surface friction conditions. The integrator may comprise a capacitor which is discharged so that its potential falls during each acceleration period from an initial predetermined level and is then charged during each intermediate period so that its potential rises, the charging rate being increased at a predetermined threshold potential below which low friction conditions are signalled.

We claim: 
     
       1. An anti-skid brake control system including sensor means that produces a first signal which is proportional to the acceleration of a braked wheel, switch means that produces a second signal and which responds to said first signal by causing a step change in said second signal at set acceleration levels so as to divide successive skid cycles into successive acceleration periods separated by intermediate periods, means for integrating said second signal so as to produce an output signal which changes in a progressive manner in one sense during each acceleration period and in the opposite sense during each intermediate period so as to vary in accordance with surface friction conditions, and output connection means whereby said output signal is used to vary a control function within the system so that the said control function is set in accordance with said surface friction conditions. 
     
     
       2. A system as claimed in claim 1 in which said integrating means is such as to commence operation by producing an output signal at a predetermined level and to change said output signal in said one sense towards a predetermined threshold level during a first acceleration period, comparison means being provided to compare said output signal with said predetermined threshold level and to produce an output signal indicative of low friction surface conditions while said threshold is exceeded. 
     
     
       3. A system as claimed in claim 2 in which said integrating means is such that the output signal changes in an exponential manner in said one sense so that it approaches a final fixed level. 
     
     
       4. A system as claimed in claim 2 in which said low friction output signal from the comparison means controls the manner in which said output signal changes in said opposite sense so that the rate of change is slower above said threshold than immediately below said threshold. 
     
     
       5. A system as claimed in claim 4 in which said output signal changes exponentially in said opposite sense, an increased rate of change occurring at said threshold level. 
     
     
       6. A system as claimed in claim 2 in which said integrating means comprises a capacitor, the potential of which corresponds to said output signal. 
     
     
       7. A system as claimed in claim 6 in which said capacitor is discharged in said one sense from a predetermined fully charged state. 
     
     
       8. A system as claimed in claim 7 in which the charging rate of said capacitor is increased at said threshold level by said output signal from the comparison means. 
     
     
       9. A system as claimed in claim 1, in which said output signal is used to vary said levels at which said step changes in said second signal are produced so that said level at which said step change is produced at the beginning of each acceleration period is reduced in accordance with said output signal. 
     
     
       10. A system as claimed in claim 1 including blanking means that produces a blanking pulse that holds said switch means in a switched state corresponding to an acceleration period, said blanking means being controlled by the output signal from said integrating means so that the duration of the blanking pulse varies with surface friction conditions, a longer blanking pulse being produced for lower friction conditions. 
     
     
       11. A system as claimed in claim 10 in which said blanking means comprises a capacitor connected in series with the input of a threshold switch to which a signal proportional to wheel speed is applied so that the switch is triggered at a wheel deceleration threshold and produces said blanking pulse whilst triggered, the charge on the capacitor controlling the length of the blanking pulse and the output signal from the integrating means controlling the rate of discharge of said capacitor.